#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
int n;
double a[10];
double f(double x)
{
	double y=-1;
	for(int i=0;i<n;i++)
		y*=x-a[i];
	return y;
}
double solve(double l,double r)
{
	while(r-l>=1e-7)
	{
		double lm=l+(r-l)/3;
		double rm=l+(r-l)/3*2;
		if(f(lm)>f(rm)){
			r=rm;
		}else{
			l=lm;
		}
	}
	return l;
}
int main()
{
	cin>>n;
	for(int i=0;i<n;i++)
		cin>>a[i];
	int p;
	cin>>p;
	cout<<fixed<<setprecision(6)<<solve(a[p],a[p+1])<<endl;
}